1. Field of the Invention
This invention relates to a process for producing transparent glass preforms from glass particles for graded-index optical fibers. More particularly, the invention relates to a process for producing such preforms which are uniform in composition and low in transmission loss.
2. Description of the Prior Art
Because of their many advantages, such as high resistance to external interference, light weight, and broad-band transmission properties, optical fibers are being used to replace the conventional copper wire that has been commercially used in the past as a medium for information transfer in the field of communications, and as means for pictorial transmission.
Two types of communication optical fibers are known: one is the clad or step-index fiber comprising a core of high refractive index and an outer cladding of low refractive index, and the other is the self-focusing or graded-index fiber which is so constructed that the index of refraction gradually lowers from the center outward. Several methods are known to produce the graded-index fiber. According to the method described in Japanese Patent Application (OPI) No. 101416/75 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a mixture of glass-forming materials and dopant are blown into a flame such as a H.sub.2 --O.sub.2 flame or a C.sub.m H.sub.n --O.sub.2 flame to form fine particles (also referred to as "soot") of glass which are deposited on a starting member to form a cylindrical particulate glass rod (porous glass) having a predetermined dopant distribution. The cylindrical particulate glass rod is then heated at a high temperature to form a transparent glass preform suitable for subsequent optical fiber-forming operations.
The conventional optical fiber preform produced by such a process often contains tiny air bubbles, which are a primary cause of reduction of the transparency of the final optical fiber and increases in the otherwise small transmission loss. A transparent preform free from air bubbles can be produced by gradually inserting the particulate glass rod into a tubular heating furnace. However, the amount of heat received by the glass rod within the furnace varies gradually along its longitudinal axis, resulting in a dopant diffusion that also varies in the longitudinal direction, and providing a preform wherein the refractive index of the periphery at one end differs from the refractive index of the periphery at the other end.